In-core-instrumentation (ICI) thimbles in several pressurized water reactor (PWR) systems have experienced excessive wear due to flow induced vibrations. This situation has led to the early and costly replacement of ICI thimbles at various plants. In a typical arrangement of a PWR with bottom mounted ICIs, the thimbles are inserted from a seal table through guide tubes and through vessel penetrations into the reactor. From there they are guided, by instrument thimble guide tubes, through the lower support structure and further on through a centrally located tube of certain fuel assemblies to the desired location and elevation within the fuel core. These ICI thimbles provide the guide path for insertion of moveable nuclear flux detectors used for reactor operation. The lower regions of fuel assemblies include very narrow annuli within the fuel assembly flow distribution plate (FDP), nozzle block or lower end fitting, where the majority of ICI thimble wear has been observed.
Several researchers have investigated the flow induced vibration aspects in this region. Gorman et al, in a paper presented in May of 1987 at the Flow-Induced Vibration Conference, BHRA, Bowness-on-Windermere, England, have attributed, for a certain geometry, the predominant flow dependent forcing mechanism to that of axial exit divergence flow from the thimble guide tube. The driving force for this flow stream is the pressure drop that exists during normal operation between the bottom of the ICI thimble guide tube (in the lower reactor vessel head region) and the top of the thimble guide tube (at the core support plate).
The invention is for use to limit wear of ICI thimble guide tubes thus extending their useful life, and is in the form of a special flow induced vibration reduction/wear-reduction-shield. It also provides an additional support leg for the fuel assembly lower end plate and thus acts as a fuel assembly anti-tipping device.